Depends on the convention. Are we talking some big professionally-produced extravaganza, or a smaller fan-run event? And a lot depends on what you want out of it. Are you going to see the big-name media stars speak, and maybe score an autograph or two? Or are you more interested in mingling and socializing with other fans? Are you interested in the dealer's room, the art show, panel discussions, or the masquerade contest? Are you into cosplay or collectibles? Are you going alone or with some like-minded friends? Or some combination thereof?
The honest truth is that there are all kinds of convention experiences out there, depending on the convention and your goals.
The only event I ever attended where an actor was the guest of honor was in 1979, when there was a one-day "Star Trek Festival" in Calgary. George Takei was the GoH.
Unfortunately, I did not get to meet him. I did get to see the two episodes that were shown (Amok Time and one other that I don't recall). George Takei gave an interesting talk, and I wandered through the dealers' area (not much on offer in 1979 for someone who was looking for books; I had everything there).
But by the time the lineup was forming for autographs, I realized that I was coming down with something that was worse than a mere cold. I hadn't been feeling very well since the night before (and of course hadn't said anything to my family; as I was still in high school, they wouldn't have let me go if they thought I was sick). So by late afternoon, I was so dizzy I could barely stand up. Turns out I had the flu, and while I can say that I attended an event and heard one of the actors speak, I completely missed getting to meet him. I've never had another chance since.
But a few years later I started going to a couple of annual conventions that took place in various cities around the province - NonCon, which was held in Edmonton, Calgary, Red Deer, and Banff. Con-Version was strictly a Calgary convention, though both cons enjoyed attendees from all over Western Canada, as well as folks from Washington and Idaho.
The major guests at these conventions were not actors. They were writers. I've had the pleasure of meeting some of the best SF novel/short story authors, and found most of them to be interesting, pleasant people. These conventions had other guests as well - other SF/fantasy authors from the US and Canada, local university professors (especially the year when one track of programming was all about writing), editors, artists, and it was a treat to have Dr. Phil Currie from the Royal Tyrrell Museum of Paleontology, to do an entire track of programming on dinosaurs and fossils. The event where Comet Shoemaker-Levy 9 hit Jupiter happened during one of the Con-Version cons, and there was a track of programming offered in partnership with the local planetarium.
These conventions featured such things as writing workshops, contests, costume contests, video rooms, authors' talks and readings, panel discussions, a dance, dealer's room, art room, filking... honestly, with all that, who has time for waiting in line for an actor's autograph for half the weekend?
That said, I will say that while I never got to meet any of the regular actors or major guest stars, I did get to meet two people who had cameos... Bjo Trimble (who had a cameo in TMP) and David Gerrold (TMP and Trials and Tribble-ations), though of course they had actually been invited as authors/BNFs.
Bjo Trimble and Sonni Cooper were guests one year, and on Sunday they issued an extraordinary invitation. They were planning to visit the Calgary Zoo on Monday, and said that if anyone wanted to join them, we could all meet up at the gate at 10 am the next morning. There were eight of us who took them up on that, and we all had a wonderful time.
So there are more kinds of conventions than just the ones where you stand in line to meet actors and fork over crazy amounts of money for autographs. The only person who ever asked for money at Con-Version for an autograph was David Gerrold - but that was for charity, so people didn't mind.
Obviously there's a huge difference between the fan-run and pro-run conventions. The 1979 one I went to was a pro-run one, and I heard stories later that it hadn't actually been that professionally-run. The others I've been to over the years have all been fan-run, and while it may sound like fan = amateur, the reality is that these were very well organized, with a good variety of guests, programming, and activities.
So when you decide to attend a convention, think first of what you really enjoy. I can't say what a large, pro-run convention is like as I've never been to one. If you pick one with a variety of activities, you should have a good time.
Some tips on surviving a convention weekend: As someone mentioned upthread, wear comfortable shoes. You're going to be spending a lot of time standing and walking around and waiting. Have water and a non-messy snack with you, as you may have a long hike or wait for room at a restaurant.
That brings up the basics. Allow time for eating, sleeping, and bathroom breaks. There's so much to do and see, and you won't enjoy it as much if you're hungry and tired. The advice given me was "eat at least once a day (regular meal) and have snacks handy, and try to get at least 4 hours of sleep."
And be prepared for weird things to happen. One year I decided to attend a panel discussion called "Are These the Dark Ages?" (intended to be a discussion of real-world issues compared with what we used to call the "Dark Ages" centuries ago). No sooner had the panel moderator announced the name of the discussion, than POOF! and the lights went out. No, it wasn't a joke... there was a fault in the electrical system, so we had a choice between canceling the discussion and continuing in the dark.
We kept going.
